Treatment of normally gaseous hydrocarbons



Nov 1, 1949. H. H. WENZKE TREATMENT OF NORMALLY GASEOUS HYDROCARBONS Filed March 27, 1943 2 Sheets-Sheet 1 A RWNNQQNK Nov 1, 1949. H. H. WENZKE 2,

TREATMENT OF NOHMALLY GASEOUS HYDROCARBONS Filed March 27, 1943 2 Sheets-Sheet 2 Patented Nov. 1, 1949 um'rsn STATES PATENT OFFICE I 2.4mm

TREATMENT OF NORMALLY GASEOUS HYDROCABBONS Application March 27, 1943, Serial N0. 480.7

(Claims. 1

This invention relates to the treatment of normally gaseous hydrocarbons, and more particularly to a method of separating individual hydrocarbons from a gaseous hydrocarbon mixture.

More specifically this invention involves the use of separation, absorption and fractionating steps connected to function in an interdependent and cooperative manner with a resultant better separation of individual hydrocarbons such as ethylene -from mixtures of normally gaseous hydrocarbons than is obtainable in equipment now in general use and operated in a conventional manner.

In one specific embodiment, the invention applies to the treatment of a mixture of essentially gaseous hydrocarbons and hydrogen such as results, for instance, from catalytic or thermal cracking processes or which may be available from conventional. gas concentration systems. The mixture is cooled in the presence of added liquid and added gases under conditions of temperature and pressure such that a maior portion of the ethylene is absorbed, leaving substantially all the lower-boiling hydrocarbon gases and hydrogen unabsorbed. The liquid containing ethylene and higher-boiling hydrocarbons from the mixture is separated from the gaseous hydrocarbons and subjected to fractionation to separate any dissolved lower-boiling hydrocarbons therefrom, .the released lower-boiling hydrocarbons being returned to be commingled with the incoming charge as said .added gases. The remaining liquid is then fractionated to separate ethylene and ethane therefrom. The ethylene and ethane are then separated by fractionation and individually removed from the system. The heavierhydrocarbons from which the ethane and ethylene have been separated are returned and contacted with the original unabsorbed low-boiling hydrocarbon gases and hydrogen, and then supplied to the normally hydrocarbon gas mixture as said added liquid. Any gases remaining unabsorbed after the last-mentioned contactingwith the recycled heavier hydrocarbons are released from the system.

At the present time, ethylene is in great demand for many process. For example, it may form the starting material for the production of alcohol, useful in the manufacture of explosives and as a starting material forbutadiene. It is also useful as one of the starting materials for various alkylation processes for the manufacture of gator fuels of exceptionally high anti-knock qua Heretoiore, processes for the separation of ethylene from the gaseous products of hydrocarbon cracking and conversion processes such as thermal cracking or reforming processes or catalytic cracking or reforming processes have not been of commercial practicability. The present invention offers an emcient process for the separation of ethene from normally gaseous hydrocarbons which is of great commercial value at this time.

The accompanying diagrammatic drawing illustrates one specific form of apparatus in which the process of the invention may be successfully conducted and the following description of the drawing includes a detailed description of the process.

Referring to the drawing. a normally gaseous hydrocarbon mixture comprised substantially of hydrocarbons boiling below butane is introduced through line I at a suitable pressure depending upon the operating temperature of the separating equipment. A gaseous medium produced as hereinafter described is conducted through line 2 and commingled with the normally gaseous hydrocarbons in line I along with a liquid medium conducted through line 3 and produced as hereinafter described. The resultant mixture is then cooled in cooler 4 to a temperature below the critical temperature of ethene which is about 49 1''. at a pressure suillcient to liquify ethene. For instance, this temperature when operating at 500 lbs. per square inch would normally be about 20 F. 0n the other hand, when operating at about 250 lbs. per square inch, this temperature would necessarily be about --20 1". The cooled mixture in line I is then directed into separation zone I wherein the liquid medium and hydrocarbons dissolved therein are separated from the unabsorbed gases. The liquid medium and dissolved hydrocarbons are withdrawn from separation zone 5 by means of line 6 and pump 1 wherefrom they areconducted by line I to stripper 9. In stripper Q, a fraction comprising a relatively small amount of ethylene and substantially all of the hydrocarbons boiling below ethylene is removed from the liquid medium and returned through line 2 to line I as the aforesaid gaseous medium. Beat for stripping may be suppliedto stripper *8 by any conventional means such as reboiler Ill.

It is, of course, within the scope of the invention to supply heat to stripper s in any wellknown manner not illustrated such as, for example, by passing the liquid therein through a heating coil, circulating a suitable heating me- 3 dium to the lower portion of the vessel or by directly commingling the liquid medium with a suitable heating medium such as steam or a reheated portion of said liquid medium.

Reboiler I is connected to stripper 9 by means of lines II and I2 and a suitable heating medium may be supplied to and withdrawn from reboiler by means of lines l3 and I4 respectively. The reboiled liquid medium substantially free from dissolved hydrocarbons boiling lower than ethylene is withdrawn from reboiler l0 by means of line I5 and pump l6 wherefrom it is directed through line l1 to fractionator IS. The function of fractionator I8 is to separate ethane and ethylene from the liquid medium, heat, therefore, being supplied to the column by means of reboiler l9 which is connected to the column by means of lines and 2|. Reboiler I9 is heated by means of a suitable heating medium supplied thereto and withdrawn therefrom by means of lines 22 and 23 respectively. The liquid medium containing substantially no ethane or lowerboiling hydrocarbons is withdrawn from reboiler l9 through line 24 to be cooled in cooler 25 wherefrom it is directed through line 26 containing valve 21 to a flash separation vessel 29. The pressure of the cooled liquid medium passing through valve 21 is reduced and the heat contained therein is sumcient to vaporize some of the lighter hydrocarbon components which are removed from vessel 28 by line 29 controlled by valve 30 to be supplied to the plant fuel gas system or used for any other purpose to which they may be suited. Unvaporized'liquid is removed from vessel 28 by means of line 3| and pump 32 from which the liquid medium is directed through line 33 containing heat exchanger 34 to condenser 35.

The above-mentioned unabsorbed gases in separation vessel 5 are removed therefrom through line 36 and directed into absorber 31. The liquid medium, after bein cooled in condenser 35, is conducted to the upper portion of absorber 31 by means of line 38. This liquid medium will dissolve a substantial portion of the gases which have been introduced to absorber 31. The liquid medium containing dissolved gases will be withdrawn from absorber 31 by means of line 39 and pump 40 wherefrom it will be directed through line 3 to be commingled with the hydrocarbon mixture in line l as the aforesaid liquid medium. Hydrocarbon gases and hydrogen undissolved in absorber 31 are removed from the system through line 4| controlled by valve 42. Some vaporous hydrocarbons are withdrawn from the upper portion of absorber 31 by means of line 43 to be commingled with the liquid medium in line 33 and condensed in condenser 35 thereby providing an added reflux for the column. Heat may be removed from an intermediate point in absorber 31 by means of pump 44 and cooler 45, which are interposed in recycle line 46.

The ethylene-ethane mixture: removed as an overhead product from fractionator I8 is directed through line 41 to condenser 48 wherefrom thecooled mixture is directed by means of line 49 to receiver 50. In receiver 50, the condensed dis tillate is separated from the gaseous material and returned by means of line 5| containing pump 52 to the upper portion of fractionator Hi to act as reflux therefor. The gaseous material is withdrawn from receiver by means of line 53 and directed into the lower portion of fractionator 54. Fractionator 54 is one section of a high efficiency fractionation system comprising fractionator 54 and fractlonator 55. The liquid The overhead product from fractionator 54 is removed therefrom by means of line 59 and cooled in condenser 60. The resultant cooled'products are then directed by means of line 5| to receiver 62, wherefrom the uncondensed gases comprising a substantial portion of ethylene are removed through line 63 controlled by valve 64. The condensed distillate is removed from receiver 52 by means of line 55 containing pump 56 and supplied to the top of fractionator 54 to act as reflux therefor. The required heat of vaporization is supplied to fractionator 55 by means of reboiler 51 connected thereto by lines 58 and 69. Reboiler 61 is heated by any suitable heating medium supplied thereto and withdrawn therefrom by means of lines 10 and 1| respectively.

The liquid product from the bottom of fractionator 55 is removed from reboiler 61 through line 12 containing valve 13. Valve 13 is regulated to cause the expansion of the liquid product from fractionator 55, thereby causing a considerable reduction in its temperature. This relatively low temperature material is then directed to heat exchanger 34 wherein it is passed in indirect contact and heat exchanger relationship with the aforesaid liquid medium carried by line 33 thus cooling the same. After passing through heat exchanger 34, the material in line 12 may be directed to the fuel gass'ystem for the plant or it may be put to any other. suitable use.

The following example is included to illustrate the utility of this invention.- In equipment such as that shown in the drawing, a normally gaseous hydrocarbon feed having an analysis as shown in the following table is charged to the plant through line I at a pressure somewhat higher than 500 pounds per square inch.

' Mol percent Hydrogen 5 2 This material is commingled .with a liquid medium retrrned through line 3 and a gaseous medium returned through line 2 and the mixture cooled in cooler 4 to a temperature of about 20 F., the pressure being substantially 500 pounds per sq'are inch. The liquid component from separator 5 is directed to stripper 9 which is operated "with a top temperature of about 40 F. and at a pressure of about 510 pounds per square inch. Reboiler I0 provides heat suflicient to maintain the bottom temperature of stripper 9 at about 137 F. The liquid fraction from stripper 9 is then directed to fractionator I8 which is operated at about 510 pounds per square inch pressure with a top temperature of about F. and a bottom temperature of about F. The overhead productfrom fractionator I8 is then treated in the fractionation system consisting of fractionators 54 and 55 which operate at a pressure of about 500 pounds per square inch, the bottom temperature of fractionator 55 being maintained at about 103 F. and the top temperature of fractionator 54 being maintained at about 22 F. The bottoms from fractionator l3 are passed through cooler 25 and pressure reducing valve 21 wherein a portion thereof is flashed thus reducing the temperature from about 100 F.. to about 58 F. The unfiashed material is then directed by means of line 3l,,

pump 32, and line 33 through heat exchanger 34, condenser 35 and line 38, into absorber 31. The bottoms from fractionator 55 are flashed by flowing through reducing valve 13 in line 12 and then passed in indirect heat exchange relation with the unfiashed bottoms from fractionator IS in heat exchanger 34. The bottoms from fractionatcr 55, after flashing, will have a temperature of about 0' F. This material in heat exchanger 34 will reduce the temperature. of the izmflashed bottoms from fractionator H! to about The overhead product from fractionator 54 is cooled in condenser 60 and the unconden'sed material removed from the system by means of line 63 controlled by valve 64. This material has an analysis as shown by the following table:

' M01 percent Methane 1.2

Ethylene 95.0

Ethane 3.8

ethylene, heavier gases, and a relatively small.

amount of the lighter gases in said liquid absorbent; separating the resultant rich absorbent from undissolved gases; stripping said rich absorbent to remove said lighter gases along with a. relatively small amount of ethylene; commingling the stripped gases with said gaseous hydrocarbon fraction and said liquid absorbent prior to said cooling step; fractionally distilling the stripped rich absorbent to separate a mixture of ethylene and ethane therefrom; fractionally distilling said ene and ethane, with said undissolved gases; and thereafter supplying said portion as said liquid absorbent in said first named commingling step.

2. The process of claim 1 further characterized in that said portion of the remaining absorbent is flash vaporized to separate light components prior to contacting with said undissolved gases.

3. A process for separating ethylene from a gaseous hydrocarbon mixture containing ethylene and lighter and heavier gases including methane and ethane which comprises commingling said gaseous hydrocarbon mixture with a liquid absorbent under superatmospheric pressure, cooling the resultant mixture to at least the liquefaction temperature of ethylene at the superatmospheric pressure employed and absorbing substantially all of the ethylene, the ethane and the heavier gases and a small amount of the lighter gases in the liquid absorbent, separating the resultant rich absorbent from the undissolved gases, stripping the lighter gases and a small amount of ethylene from the hydrocarbon mixture and said liquid absorbent prior to the coollast named mixture to separate relatively pure ethylene; contacting at least a portion of the remaining absorbent, substantially free of ethyling step, fractionally distilling the stripped rich absorbent to separate a mixture of ethylene and ethane therefrom, fractionating said mixture of sorbent thus freed of dissolved gases with said undissolved gases to remove therefrom ethylene and heavier gases not dissolved in the cooling step and thereafter supplying said portion as said liquid absorbent in said first-named commingling I step.

4. The process of claim 3 further characterized in that the ethane separated during the fractionation of said mixture of ethylene and ethane is removed under pressure in the liquid phase from the fractionating step, the pressure released and the resultant cooled vaporized ethane passed in indirect heat exchange relationship with the absorbent freed of dissolved gases prior to contacting the latter with said imdissolved gases.

REFERENCES CITED The following references are of record in the flle of this patent:

UNITED STATES PATENTS 

